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Fix race with shared tag queue maps

PATCH In review Jens Axboe 3 participants 6 messages Sep 13, 2007 – Sep 14, 2007

Subsystems (2)
SCSI Block layer
lore
Share & tools
lore

Jens Axboe

author
#0 lore raw

PATCH 1/1 Fix race with shared tag queue maps

Hi,

There's a race condition in blk_queue_end_tag() for shared tag maps, users include stex (promise supertrak thingy) and qla2xxx. The former at least has reported bugs in this area, not sure why we haven't seen any for the latter. It could be because the window is narrow and that other conditions in the qla2xxx code hide this. It's a real bug, though, as the stex smp users can attest.

We need to ensure two things - the tag bit clearing needs to happen AFTER we cleared the tag pointer, as the tag bit clearing/setting is what protects this map. Secondly, we need to ensure that the visibility of the tag pointer and tag bit clear are ordered properly.

This is for 2.6.23-rc6-current, but it needs to go into -stable as well once Linus has committed it. I'm cc'ing users that reported stex problems, hopefully they can test this patch and report back.

Also see http://bugzilla.kernel.org/show_bug.cgi?id=7842

Signed-off-by Jens Axboe <[email protected]>
block/ll_rw_blk.c +17 -6
index a15845c..3d9e6a1 100644
@@ -1075,12 +1075,6 @@ void blk_queue_end_tag(struct request_queue *q, struct request *rq)
 		 */
 		return;
 
-	if (unlikely(!__test_and_clear_bit(tag, bqt->tag_map))) {
-		printk(KERN_ERR "%s: attempt to clear non-busy tag (%d)\n",
-		       __FUNCTION__, tag);
-		return;
-	}
-
 	list_del_init(&rq->queuelist);
 	rq->cmd_flags &= ~REQ_QUEUED;
 	rq->tag = -1;

@@ -1090,6 +1084,23 @@ void blk_queue_end_tag(struct request_queue *q, struct request *rq)
 		       __FUNCTION__, tag);
 
 	bqt->tag_index[tag] = NULL;
+
+	/*
+	 * Ensure ordering with tag section
+	 */
+	smp_mb__before_clear_bit();
+
+	if (unlikely(!test_and_clear_bit(tag, bqt->tag_map))) {
+		printk(KERN_ERR "%s: attempt to clear non-busy tag (%d)\n",
+		       __FUNCTION__, tag);
+		return;
+	}
+
+	/*
+	 * Ensure ordering between ->tag_index[tag] clear and tag clear
+	 */
+	smp_mb__after_clear_bit();
+
 	bqt->busy--;
 }
Signature · 1 line
Jens Axboe
#1 lore raw

block/ll_rw_blk.c void blk_queue_end_tag(struct request_queue *q, struct request *rq) · line 1093

Quoted 7 lines · Jens Axboe
block/ll_rw_blk.c +7 -0
+
+	/*
+	 * Ensure ordering with tag section
+	 */
+	smp_mb__before_clear_bit();
+
+	if (unlikely(!test_and_clear_bit(tag, bqt->tag_map))) {

You don't need the "smp_mb__before_clear_bit()" there.

The regular "clear_bit()" needs it, but the "test_and_xxx()" versions are architecturally defined to be memory barriers, exactly because they are regularly used for locking.

This is even documented - see Documentation/atomic_ops.txt.

Linus

author
#2 lore raw
Quoted 13 lines · Linus Torvalds

Review

Quoted 7 lines · Jens Axboe
(quoted diff) +7 -0
+
+	/*
+	 * Ensure ordering with tag section
+	 */
+	smp_mb__before_clear_bit();
+
+	if (unlikely(!test_and_clear_bit(tag, bqt->tag_map))) {

You don't need the "smp_mb__before_clear_bit()" there.

Quoted 5 lines

The regular "clear_bit()" needs it, but the "test_and_xxx()" versions are architecturally defined to be memory barriers, exactly because they are regularly used for locking.

This is even documented - see Documentation/atomic_ops.txt.

My bad, I think I added the smp_mb__before_clear_bit() when it was __test_and_set_bit() like in the first hunk.

block/ll_rw_blk.c +12 -6
index a15845c..dfc9f22 100644
@@ -1075,12 +1075,6 @@ void blk_queue_end_tag(struct request_queue *q, struct request *rq)
 		 */
 		return;
 
-	if (unlikely(!__test_and_clear_bit(tag, bqt->tag_map))) {
-		printk(KERN_ERR "%s: attempt to clear non-busy tag (%d)\n",
-		       __FUNCTION__, tag);
-		return;
-	}
-
 	list_del_init(&rq->queuelist);
 	rq->cmd_flags &= ~REQ_QUEUED;
 	rq->tag = -1;

@@ -1090,6 +1084,18 @@ void blk_queue_end_tag(struct request_queue *q, struct request *rq)
 		       __FUNCTION__, tag);
 
 	bqt->tag_index[tag] = NULL;
+
+	if (unlikely(!test_and_clear_bit(tag, bqt->tag_map))) {
+		printk(KERN_ERR "%s: attempt to clear non-busy tag (%d)\n",
+		       __FUNCTION__, tag);
+		return;
+	}
+
+	/*
+	 * Ensure ordering between ->tag_index[tag] clear and tag clear
+	 */
+	smp_mb__after_clear_bit();
+
 	bqt->busy--;
 }
Signature · 1 line
Jens Axboe
#3 lore raw
Quoted 3 lines · Jens Axboe

My bad, I think I added the smp_mb__before_clear_bit() when it was __test_and_set_bit() like in the first hunk.

Ahh, that wouldn't work at all. The "__test_and_set_bit()" thing isn't atomic at all, and no amount of memory barriers around it would help (you'd need to use real locking, but at that point the memory barriers are pointless anyway).

block/ll_rw_blk.c void blk_queue_end_tag(struct request_queue *q, struct request *rq) · line 1097

Quoted 4 lines
block/ll_rw_blk.c +4 -0
+	/*
+	 * Ensure ordering between ->tag_index[tag] clear and tag clear
+	 */
+	smp_mb__after_clear_bit();

You still left this one. But never mind - I already edited your original patch and it's in my tree with both of those things removed.

Linus

author
#4 lore raw
Quoted 11 lines · Linus Torvalds
Quoted 3 lines · Jens Axboe

My bad, I think I added the smp_mb__before_clear_bit() when it was __test_and_set_bit() like in the first hunk.

Ahh, that wouldn't work at all. The "__test_and_set_bit()" thing isn't atomic at all, and no amount of memory barriers around it would help (you'd need to use real locking, but at that point the memory barriers are pointless anyway).

Hence the change, it looks like an oversight from when we didn't allow sharing of tag maps (then the queue lock provided adequate protection).

Quoted 7 lines

block/ll_rw_blk.c void blk_queue_end_tag(struct request_queue *q, struct request *rq) · line 1097

Quoted 4 lines
block/ll_rw_blk.c +4 -0
+	/*
+	 * Ensure ordering between ->tag_index[tag] clear and tag clear
+	 */
+	smp_mb__after_clear_bit();

You still left this one. But never mind - I already edited your original patch and it's in my tree with both of those things removed.

I took at look at the committed patch and it looks fine, thanks Linus.

Signature · 1 line
Jens Axboe
#5 lore raw
In reply to Jens Axboe
Quoted 4 lines · Jens Axboe

Hi,

There's a race condition in blk_queue_end_tag() for shared tag maps, users include stex (promise supertrak thingy) and qla2xxx.

[...]

Quoted 2 lines

I'm cc'ing users that reported stex
problems, hopefully they can test this patch and report back.

I'm running 2.6.22 with f3da54ba140c6427fa4a32913e1bf406f41b5dda patch from Linus git tree (instead of Ed Lin workaround patch) and doing heavy rsync for last 8 hours. No oops - seems to be working fine (+stex driver).

Thanks!

Signature · 6 lines
Arkadiusz Miśkiewicz        PLD/Linux Team
arekm / maven.pl            http://ftp.pld-linux.org/
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